The present invention relates to a continuous manufacturing process for metallic strips, such as steel strips, in which at least one face is covered with a sub-layer of tin and an upper layer of organic coating.
The invention applies particularly to the manufacturing of steel strips for packaging such as food cans and beverage cans.
In one of the known manufacturing procedures for this type of strips of steel, the steel is tin plated electrolytically, then coated with an organic coating by coating with a lacquer either using a brush or a roller.
After coating, the lacquer, which is essentially composed of a duroplastic polymer and an organic solvent, undergoes curing to initiate or complete the polymerizing and to create the tin-polymer bonds that make it possible for the lacquer to adhere to the tin-plated steel. The difficulty lies in control of the curing temperature. In fact, in the case of undercuring of the lacquer, adequate tin-polymer bonds are not formed or they may even remain capable of being hydrolyzed (presence of hydrogen bonds) and there is a risk of the appearance of separation at the time that the cans manufactured with this type of coated steel are sterilized. In summary, in the case of undercuring, a loss of adhesion of the lacquer on the tin-plated steel is observed and/or a discoloration (browning) of the lacquer.
In addition to having to strictly control the curing of the lacquer, this procedure has other disadvantages, mentioned below:
it is absolutely necessary to apply the lacquer coating in a thin layer in order to facilitate total elimination of the organic solvent, of the water and more generally of the different volatile products of polymerization; this makes it necessary to have successive lacquer coatings on the tin-plated steel strip with an adequately slow speed on the order of 100 m/min. for production line lacquering, and 20 m/min. for sheet lacquering to obtain a lacquer layer with satisfactory thickness; generally, the coating proceeds with two to four layers of lacquer to obtain a final layer with a thickness on the order of 8 to 25 xcexcm;
successive application of several lacquer layers requires undercuring of the first layers to avoid their overcuring at the time of thermoprocessing of later layers, which has a negative effect on the adhesion of the coating and its visual appearance; this considerably complicates controlling the adhesion of the lacquer on the substrate;
currently, environmental regulations impose standards that are more and more restrictive in view of reducing the use of organic solvents (VOC), and even eliminating them, which means that industries have to try to find replacement solutions;
the cost of the lacquers is very high and their usage is limited in time because of the storage problems that they present; in fact, at the time of storage, there is the risk of observing a partial or total polymerization in the can, which makes the lacquer useless;
the cost of construction for a tin-plating line is very high.
In another procedure, the steel that is previously degreased, pickled and dried is tin plated with an electrolytic deposit of tin, then coated with a thermoplastic film by bonding the film on the tin layer with an adhesive.
This procedure also involves disadvantages. In fact, the adhesion of the polymer on the steel or on the tinned sheet (steel coated with tin or even tin-plated steel) is not instantaneous. Thus, this is fairly difficult to obtain in a fraction of a second of contact, even if the polymer is melted and hot. In addition, management of a melted layer is a problem that has not yet been resolved for polymers of the polyethylene terephthalate type, PET. Besides that, the adhesion, which itself is fairly difficult, requires the elimination of solvents from the adhesive through the polymer film, which is not easy. It should be noted, in addition, that the presence of the adhesive between the steel and the polymer film can easily be detected by the infrared technique.
Thus the goal of the present invention is to propose a procedure for continuous manufacturing, at high speed, of strip steel of which at least one face is coated with a sub-layer of tin and an upper layer of an organic coating having good adhesion to the substrate, which makes it possible to better control the thickness of the organic coating by application of a polymer layer, all at once, that is possibly very thick and to avoid use of organic solvents or adhesives.
For this purpose, the object of the invention is a process for manufacturing coated metallic strip for packaging made up of a thin metallic strip coated on at least one of its faces with a coating having, in order starting from the surface of the thin metallic strip, a layer of tin or tin base alloy, possibly an intermediate metallic layer and an organic layer formed of a polymer film, according to which:
on one hand, on at least one of the faces of the thin metallic strip carrying out tin dip coating or tin plating places a layer of tin or tin base alloy;
on the other hand, on one face of at least one film of polymer, at least one metallic layer is vacuum deposited in such a way as to obtain a polymer film coated on one face with an external layer of tin or tin base alloy;
then at least one film of coated polymer and the thin metallic strip having at least one layer of tin or tin base alloy are brought together in such a way that the face of the at least one film of polymer covered with tin or tin base alloy is across from one face of the thin metallic strip having a layer of tin or tin base alloy, by injecting between the two faces a flux to clean the surfaces covered with a layer of tin or tin base alloy; and
at least one polymer film is welded to the face of the metallic strip.
The procedure according to the invention may comprise one or several of the following characteristics:
the thin metallic strip is of steel;
at least one polymer film is coated on one of its faces, first with an intermediate layer of aluminum or aluminum base alloy, then a layer of tin or tin base alloy;
the at least one film of polymer is welded on the thin metallic strip using hot rolling at a temperature between the melting temperature of the tin or tin base alloy and the melting temperature of the polymer;
to carry out the hot rolling, the thin metallic strip is heated by induction or conduction;
the basis weight of the layer of tin on the steel strip after tin plating or tin coating is between 0.05 and 1 g/m2, preferably between 0.1 and 0.2 g/m2;
the polymer film is a thermoplastic polymer film preferably chosen from among PET, PAN or PTFE, of which the thickness is between 5 and 10 xcexcm, preferably between 10 and 20 xcexcm;
the vacuum depositing of the tin or tin base alloy layer on the polymer film is carried out using PVD, and the thickness of this layer of tin or tin base alloy is between 0.01 and 1 xcexcm, preferably between 0.05 and 0.2 xcexcm;
the flux is carried out using an alcohol-rosin dispersion, phenolsulfonic acid or chlorhydric acid, simultaneously on the tin or tin base alloy covering the steel strip and on the tin or tin base alloy covering the polymer film.
Finally, one goal of the invention is coated metallic strip for packaging making up strip steel covered on at least one of its faces with a coating comprising, in the order starting from the surface of the steel strip, a layer of tin or tin base alloy, possibly an intermediate aluminum or aluminum base layer, and a film of thermoplastic polymer without a trace of adhesive between the layer of tin or tin base alloy and the thermoplastic polymer film.
As would be understood, the invention consists of welding the tin on itself to avoid the problems of undercuring or overcuring inherent with duroplastic polymers making up the lacquer and avoiding the use of adhesive.
This procedure can be applied to any polymers, under the condition that their melting point is greater than the melting temperature of tin (232 C.) or of the tin base alloy, depending on the case.